The present invention relates to a device for measuring the diameters of particles.
A number of well known devices have been used in carrying out the so-called forward scattering method of measuring the diameters of particles, such drops of liquid in the turbine (the diameter of each of drops being from 0.1 to 10 .mu.m) and drops of fuel which are jet-sprayed into the combustion means (the diameter of each of drop being from 2 to 200 .mu.m).
Particle diameter measurement according to the forward scattering method is based on the following principle. Where parallel beams of mono-chromatic light, such as laser beams, are irradiated onto a spherical particle whose diameter is D, the light intensity scattered by one particle in a direction which makes an angle .theta. in relation to the incident direction of the beam can be calculated accurately, based on the Mie scattering theory. On the other hand, where laser beams are irradiated onto a number of particles, the measured light intensity I(.theta.) scattered by many particles to the angle .theta. is expressed by the following equation (1): EQU I(.theta.)=.intg.i(D,.theta.).multidot.n(D)dD (1)
wherein n(D) represents the density of particles whose diameter is D. When the light intensity scattered by a number of particles is measured at various angles .theta. to obtain the angular distribusion I(.theta.) of scattered light intensity, the particle diameter distribution n(D) can be calculated from this distribution I(.theta.), employing equation (1).
FIG. 1 shows a conventional particle diameter measuring device designed to carry out the forward scattering method. Laser beams 14 emitted from a laser 10 pass-through collimater lenses 12 to become parallel beams having an adequate diameter, which are then irradiated onto a group of particles 16 to be measured, and are scattered by these particles. A photo-detector array 20 is separated only by the focusing distance f of a collecting lens 18 therefrom. Scattered light are received by the photo-detector array 20 through the collecting lens 18. The photo-detector array 20 comprises a plurality of ring-shaped photosensitive elements arranged concentrically on a face. The light scattered at angle .theta. is received by a photosensitive element whose radius is r(=.theta.f), and its light intensity is converted into an electric signal. Signals detected by the photo-detector array 20 are amplified by an amplifier 22 and are then fed to an arithmetic unit 24, where the particle diameter distribution n(D) is calculated from equation (1), using the angular distribution of scattered light intensity I(.theta.) detected by the photo-detector array 20.
The conventional device, however, cannot measure scattered light intensity in a wide angle range, because the maximum aperture and minimum focal length for the collecting lens 18 are limited. The measuring range of the angle distribution I(.theta.) of scattered light intensity conducted by the conventional device is thus limited to the range from 0 to about 15.degree.. FIG. 3 shows the light intensity i(D,.theta.) scattered by each of the particles whose diameters D are 0.10, 0.20, 0.50, 1.00, 2.00 and/or 100.00 .mu.m. In the case of large-diameter particles, the distribution patterns of the light intensity i(D,.theta.) scattered by the particle may be distinguishable, even if the range of the measuring angles of the angular distribution I(.theta.) of scattered light intensity is narrow. Therefore, the particle diameter distribution n(D) can be obtained from the results of the narrow angular distribution I(.theta.) measurements. In the case of small-diameter particles, however, it is necessary to measure the angular distribution I(.theta.) of scattered light intensity over a wide range of scattering angles to distinguish the light intensity distribution scattered by the particle of a specified diameter and that scattered by the particles of other diameters, since the period of which the light intensity i(D,.theta.) scattered by the particle changes in relation to the scattering angle .theta. is long. Therefore, the conventional device can not obtain the particle diameter distribution n(D) of small-diameter particles, particularly particles whose diameters D are smaller than 1 .mu.m. In addition, since high intensity transmitted light is irradiated onto a central portion 20a of the photo-detector array 20, high current is yielded in the central portion 20a and leaks to the circumferential ring-shaped photosensitive elements, to cause errors in the measurement of the angular distribution I(.theta.) of the scattered light intensity.